Aircraft electrical power systems have increasingly required the development of systems which can provide larger and larger electrical supplies under both normal and abnormal conditions. As noted in U.S. Pat. No. 6,467,725 issued to Coles et al., the entire disclosure of which is incorporated herein by reference, a greater dependence on electrical power requires an electrical supply that is available at all times that an aircraft is in service.
As noted in the Coles Patent, emergency electrical power in the event of engine failure has been traditionally provided by auxiliary devices, such as a ram air turbine (RAT), which comprises an electrical generator equipped with a propeller. The RAT is normally stored within the fuselage of an engine and is deployed into the air stream surrounding the aircraft when required. The resulting flow of air over the RAT causes a propeller to rotate, thereby generating electrical power. Unfortunately, several such devices may be required in order to ensure sufficient power is available in the event of total engine failure which can incur significant additional weight to the system.
The Coles Patent addresses the problems disclosed above through the incorporation of a “windmill” effect applied to a bypass fan and a generator. As pointed out in the Coles Patent, it is known that in a multistage high bypass gas turbine engine, the low pressure shaft (LP) or low speed spool which drives the low pressure compressor and the bypass fan will continue to rotate in the event of engine failure because of a “windmill” effect created by the airflow resulting from the motion of the aircraft. The energy of the fan is then extracted by a generator connected to the low pressure shaft, and is then used to supply electrical power to the aircraft during periods of failure.
Where engine failure does not occur, of equal concern is the failure of systems or components in the power generation system. Such a power generation system typically includes a permanent magnet generator, an exciter salient-pole synchronous machine, and a main salient-pole synchronous machine coupled with a prime mover, such as a gas turbine engine. During normal operation, the permanent magnet machine provides rectified power to an exciter regulator, which in turn controls an exciter field current that produces a rotating multi-phase voltage. This multi-phase voltage is rectified to produce a main machine field current and the resultant flux produced by this field current produces a voltage at the stator windings of the main machine. Regulation of the output voltage is achieved by comparing the output voltage to a pre-determined reference and adjusting the exciter field winding.
However, the loss of excitation power to the main machine can be caused by the failure of any system or component noted above, including the armature of the PM generator, the diodes of the multiphase rectifier which serve to rectify the output of the armature voltages of the PM generator, the exciter regulator (i.e. semiconductor switches), the generator control unit (GCU), the field and armature of the exciter machine, the diodes of the multiphase rotating rectifier, and the field winding of the machine.
The loss of excitation prevents the production of a main field flux in the main machine, and as a consequence, the machine can not produce any electrical power. Accordingly, there is a need for a system and method to control a power generation system to provide at least partial power generation capability by using a reluctance machine generally, and in a specific example, by using the reluctance power of the salient-pole synchronous generator where the prime mover is operational but the main field flux is lost. Furthermore, there is also a need for a system and method to simplify the traditional generation system by utilizing only one electrical machine which can eliminate the need for other electrical machines such as PM and exciter machines of the traditional aircraft generation system.
Accordingly, a need exists for a system and method which enables DC power generation from a synchronous reluctance machine, including the specific example of a salient-pole synchronous machine where the prime mover is operational but excitation is lost (essentially resulting in a synchronous reluctance machine).